One way to help out with this problem is using a different hardener for higher temp environments. Also, instead of mixing in a cup or other small container, mix on a board and spread out the epoxy. This will allow the heat of the chemical reaction to disperse more. When the epoxy cures, it creates heat. Heat adds more energy to the mixture causing it to catalyze faster. It's a snowball effect.

If you are going to chill the mixture, chill the parts before you mix them. I keep a resin and hardener in my fridge for summer repairs.

For more detailed information on epoxy check out these guys.

CLC Boats

They really know thier stuff. Though most of thier problems are dealing with the epoxy being too cold during the winter.